A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may for example project a pattern from a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate.
The wavelength of radiation used by a lithographic apparatus to project a pattern onto a substrate determines the minimum size of features that can be formed on that substrate. A lithographic apparatus that uses EUV radiation, being electromagnetic radiation having a wavelength within the range 4-20 nm, may be used to form smaller features on a substrate than a conventional lithographic apparatus (which may for example use electromagnetic radiation with a wavelength of 193 nm).
A mask that is used to impart a pattern to a radiation beam in a lithographic apparatus may form part of a mask assembly. A mask assembly may include a pellicle that protects the mask from particle contamination. The pellicle may be supported by a pellicle frame.
The use of pellicles in lithography is well-known and well-established. A typical pellicle in a DUV lithographic apparatus is a membrane which is located away from the mask and is out of the focal plane of a lithographic apparatus in use. Because the pellicle is out of the focal plane of the lithographic apparatus, contamination particles which land on the pellicle are out of focus in the lithographic apparatus. Consequently, images of the contamination particles are not projected onto the substrate. If the pellicle were not present, then a contamination particle which landed on the mask would be projected onto the substrate and would introduce a defect into the projected pattern.
It may be desirable to use a pellicle in an EUV lithographic apparatus. EUV lithography differs from DUV lithography in that it is typically performed in a vacuum and the mask is typically reflective rather than being transmissive. Challenges relating to the use of a pellicle for EUV lithography may arise which are not present when a pellicle for DUV lithography.
It may be desirable to provide a mask assembly and associated methods which overcome or mitigate a problem associated with the prior art.